Since x-rays were first discovered, dental radiology has seen several milestones. These include the development of panoramic radiography, dental tomography, dental tomosynthesis, more efficient image detectors, computerized dental radiography and, most recently, cone-beam computed tomography (CBCT). CBCT has found wide acceptance in the dental community because of its ability to provide a three-dimensional (3D) representation of the hard tissues of the oral and maxillofacial region. While CBCT has been shown useful for many applications, it has not shown to improve caries detection. In part, this is the result of the beam hardening artifacts that occur in teeth. These artifacts may obscure existing caries lesions or may induce false-positive diagnoses. Even a high-resolution CBCT system with a small field-of view was shown not to improve the detection of proximal caries lesions.
Intraoral radiography is the mainstay of dental imaging. It provides relatively high resolution, and limited field of view images for most routine dental needs. However, as a two-dimensional (2D) imaging modality, the technique suffers from superimposition of overlying structures and loss of spatial information in the depth dimension. Panoramic imaging, a popular form of extra-oral imaging, visualizes the entire maxilla, mandible, temporo-mandibular joints (TMJ) and associated structures in a single image, but it is subject to considerable geometric distortion and has relatively low spatial resolution compared with intraoral radiography. CBCT as a three-dimensional (3D) imaging modality has found wide acceptance in dentistry, especially for surgical planning procedures such as dental implant and orthodontic treatment planning, and evaluation of endodontic and pathological condition. There are, however, several disadvantages associated with CBCT in comparison to 2D radiography: (1) excess noise and artifacts from metal dental restorations/appliances reduce the image quality; (2) acquisition, reconstruction, and interpretation time are greatly increased, reducing clinical efficiency and increasing financial cost; and (3) significantly higher ionizing radiation doses increase radiation burden for the patient.
Despite the many technological advances, the radiographic diagnostic accuracy for some of the most common dental conditions has not improved in many years and in some cases remains low. Examples include caries detection, root fracture detection, and assessment of periodontal bone loss.
Caries is the most common dental disease. The World Health Organizations estimates that 60-90% of school children and nearly all adults have dental caries at some point in time. If carious lesions are detected early enough, i.e. before cavitation, they can be arrested and remineralized by non-surgical means. When carious lesions go undetected, they can evolve into more serious conditions that may require large restorations, endodontic treatment, and, in some cases, extractions. The detection sensitivity of caries has not seen any significant improvement in the past several decades. 2D intraoral radiography is the current gold standard, with a reported sensitivity ranging from 40% to 70% for lesions into dentine and from 30% to 40% for lesions confined to enamel. CBCT does not provide significant improvement for caries detection. Beam-hardening artifacts and patient movement decrease structure sharpness and definition.
The detection of vertical root fractures (VRF) represents a clinically significant diagnostic task with important ramifications in tooth management. VRFs are considered one of the most frustrating tooth conditions associated with endodontic therapy. Overall detection of VRFs remains poor. The ability of CBCT to detect initial small root fractures is limited by its relatively low resolution. Furthermore, excess beam hardening, streak artifact, and noise result in both significantly decreased sensitivity and increased false positive root fracture diagnosis.
Dental radiography provides important information for assessing tooth prognosis and making treatment decisions associated with periodontal disease. Conventional 2D intraoral radiography provides exceptionally high image detail of key dental structures, but because of structure superimposition delivers poor assessment of alveolar bone architecture and consistently underestimates bone loss. CBCT conversely delivers more accurate 3D assessment of clinically-relevant morphologic alveolar bone defects but with a penalty in image detail. Beam hardening and streak artifacts are a significant problem for accurate bone morphology characterization.
These diagnostic tasks illustrate the clinical need for a diagnostic imaging system with high resolution, 3D capabilities, reduced metal artifact and lower radiation burden to patients.
Digital tomosynthesis imaging is a 3D imaging technique that provides reconstruction slice images from a limited-angle series of projection images. Digital tomosynthesis improves the visibility of anatomical structures by reducing visual clutter from overlying normal anatomy. Some examples of current clinical tomosynthesis applications include chest, abdominal, musculoskeletal, and breast imaging. For example, an industrial prototype extra-oral tomosynthesis scanner may use a dental x-ray tube operating at 60 peak kilovoltage (kVp) and 7 milliampere (mA), and an exposure time of 0.4 seconds (s) for each projection image. The scanning time may be 40 s. A set of 24 radiographs may be taken in a circular scanning geometry with a total of 22 degrees (°) coverage. A system resolution may be 5 lines per millimeter (lp/mm) and a reconstruction slice thickness may be 1 mm. A total absorbed dose for the 24 projections may be 16 milligray (mGy), corresponding to a maximum body dose of 16 millisievert (mSv), close to that from a panoramic scan of 12 mGy. Notably, such a dose is lower than that used in CT. In this manner, it may be concluded that tomosynthesis offers higher spatial resolution and less radiation compared to CT and is less expensive.
Digital tomosynthesis has been applied to dental imaging in the past. A variation of the tomosynthesis technique, called Tuned Aperture Computed Tomography (TACT), was investigated in the late 1990's for dental imaging. TACT significantly improved the diagnostic accuracy for a number of tasks compared to conventional radiography. These included: (1) root fracture detection, (2) detection and quantification of periodontal bone loss, (3) implant site assessment, and (4) the evaluation of impacted third molars. The results for caries however were inconclusive.
TACT was not adopted clinically because the technology was not practical for patient imaging. Conventional x-ray tubes are single pixel devices where x-rays are emitted from a fixed point (focal spot). To acquire the multiple projection images, an x-ray source was mechanically moved around the patient. A fiduciary marker was used to determine the imaging geometry. The process was time consuming (e.g., approximately 30 minutes per scan) and required high operator skill to accomplish image acquisition.
Intraoral tomosynthesis imaging has also been considered. For example, a hand-held tuned-aperture computed tomography system with an intraoral x-ray detector, such that a dental x-ray source is mechanically moved to different locations with respect to the detector to produce multiple projection images for the reconstruction may be utilized. In such a case, a fiducial marker is used to track the positions of the x-ray source relative to the detector. Alternatively, an intraoral tomosynthesis scanner with a conventional dental x-ray tube mounted on a mechanical rotating gantry may also be used. The tube is mechanically moved to the different locations along the arc to generate the different projection images.
Compared to intraoral systems, extra-oral devices require higher radiation exposure for the patients, an image quality that is affected by out-of-focus structures due to incomplete blurring of an opposing jaw, and significant beam hardening artifacts. For example, extra-oral tomosynthesis has been investigated in a patient study using an experimental device, and using CBCT. The extra-oral geometry required high radiation dose. The image quality was compromised by cross-talk of out-of-focus structures. Intraoral tomosynthesis using a single mechanically scanning x-ray source has been described in the patent literature, and investigated in a recent publication using a single conventional x-ray source and a rotating phantom.
One common limitation of the current tomography system including tomosynthesis is that the x-ray source and/or the detector must be moved to different viewing angles to generate the projection views needed for reconstruction. This is because most of the conventional x-ray tubes contain either one or two focal spots where the x-ray radiation is emitted. To illuminate the object from different directions, the source, the detector, or the object needs to be rotated or translated. The mechanical motion of the gantry limits the scanning speed and resolution, and thereby makes the system large and less mobile.
Spatially distributed x-ray arrays with multiple x-ray focal spots are now commercially available from manufacturers including XinRay Systems. These source arrays can electronically generate a scanning x-ray beam from different locations or focal spots without any mechanical motion.
One example is the carbon nanotube based field emission x-ray source array that utilizes an array of individually controllable electron emitters such as the carbon nanotubes (CNTs) as the ‘cold cathodes’ to generate electrons which are accelerated to bombard the anode to produce x-ray radiations. By electronically switching on and off the individual CNT cathodes, a scanning x-ray beam is generated from each different focal spots on the x-ray anode.
The CNT x-ray source array is particularly attractive for tomography imaging. By generating projection views from different angles without mechanical motion, it enables stationary tomography with faster scanning speed and higher resolution. The flexibility in the source configuration also opens up the feasibility of designing tomography scanners with novel geometries.
A stationary digital breast tomosynthesis (s-DBT) system has been disclosed in U.S. Pat. No. 7,751,528 and the Chinese application 200880107680.X “Stationary x-ray digital breast tomosynthesis systems and related methods”, incorporated by reference herein, which also discloses the general concept of stationary digital. The stationary design increases the system spatial resolution by eliminating the image blurring caused by x-ray tube motion. A faster scan time is achieved by integrating with a high-frame-rate detector to minimize patient motion and discomfort under compression. The stationary design without the constraint of mechanical motion also allows a wider angle tomosynthesis scan for better depth resolution without changing the scanning time.
Additionally, feasibility of stationary chest tomosynthesis has recently been investigated using a linear CNT x-ray source array.